Attempting to install the relevant ISO 2- and 3-letter codes as the enumerated possible values is probably never going to be a realistic possibility. See RFC 3066 at http://www.ietf.org/rfc/rfc3066.txt and the IANA registry at http://www.iana.org/assignments/lang-tag-apps.htm for further information. The union allows for the 'un-declaration' of xml:lang with the empty string.
Source
<xs:element name="evaluationDetails"><xs:annotation><xs:documentation xml:lang="en">Provides further information on the evaluation process of a tool or service</xs:documentation><xs:appinfo><identifier>http://w3id.org/meta-share/meta-share/evaluationDetails</identifier><label xml:lang="en">evaluation details</label></xs:appinfo></xs:annotation><xs:complexType><xs:simpleContent><xs:restriction base="ms:langString"><xs:maxLength value="500"/></xs:restriction></xs:simpleContent></xs:complexType></xs:element>